The present invention relates to an analog-to-digital (hereinafter often simply referred to as A/D) converter that converts an analog voltage signal into a corresponding digital signal and, more specifically, to an A/D converter of the over-sampling type that is suited for being realized in the form of a semiconductor integrated circuit.
As an example of the A/D converters there is an over-sampling type which uses sampling frequencies which are several tens to several hundreds of times as high as the frequency band of the analog input signals. The A/D converters of the over-sampling type can be classified into several types depending upon their circuit constitutions, such as those of the delta-sigma (.DELTA..SIGMA.) type disclosed in a first literature ISSCC '85, Digest of Technical Papers, pp. 80-81 (February 1985) and those of the interpolation type disclosed in a second literature ISSCC '86, Digest of Technical Papers, pp. 180-181 (February 1986).
A system which converts an input analog voltage signal into a current signal by using a voltage-current current converting circuit has been taught in a third literature IEEE Journal of Solid-State Circuits, SC-21 (December 1986) pp. 1003-1010.